@AmmoJammo: when you are in 68k mode is your fast ram on the MMR (assuming there is some) still available? That is probably the problem, I had similar problems with my A590 when I had an mtec 68020 accelerator with fast ram... the problem seemed to be that the fast ram was mapped into DMA space or flagged as DMA capable which caused the A590 to try to use it for DMA, which doesn't work.
The best I could manage until I went with a different HD controller was to boot from floppy and mount the HD manually via a mountlist and then boot the HD.
I guess when in 030 mode the ram is mapped out of DMA space.
Disclaimer: It's been many years since I had this gear so I might be misremembering some details!
I guess when in
